This invention relates to a process for continuous preparation of aromatic fluorides wherein corresponding aromatic diazonium fluorides are continuously decomposed at low concentration in a reaction system including one or more continuous flow through cascaded reactors.
Aromatic fluorides prepared in accordance with the present invention are useful as intermediates in preparing various pesticides, pharmaceuticals and other products.
Processes have heretofore been proposed for preparation and further reaction of aromatic diazonium salts as well as preparation of fluoroaromatics involving decomposition of corresponding diazonium fluorides.
Kindler, U.S. Pat. No. 3,423,391, discloses a process for the continuous diazotization, particularly at relatively high temperatures, of amines with diazotizing agents and for the further reaction of the resultant diazo compounds or allegedly for their recovery as such. According to the patent, further processing of the diazo compounds thus formed into azo dyes, phenols, hydrazines or other compounds capable of being prepared therefrom may also be carried out, preferably continuously, in reactors of conventional types, for example in flow tubes, agitated vessels or a cascade of such vessels or a circulation reactor. The patent discloses that the diazo compounds obtained are separated or further reacted so rapidly that no appreciable amounts of decomposition products can form. According to the patent, substantially all (e.g., at least 50%) of the heat of reaction from the continual mixing and reacting of the amine and the diazotizing agent is absorbed by the reaction mixture, the temperature thereof rising by from 20.degree. C. to 50.degree. C. or more, and the diazo compounds formed are obtained at 40.degree. C. to 80.degree. C. or, in case of high melting point or sparingly soluble components, to 100.degree. C. or even higher.
Japanese Patent Publication No. 49[1974]-81330, published Aug. 6, 1974, discloses a method for preparation of aromatic fluoro componds wherein a hydrofluoric acid solution of a compound that gives nitrous acid is allowed to act on a substituted or unsubstituted aromatic amino compound and diazotization and thermal decomposition of the diazonium salts produced are carried out in one step at a reaction temperature that causes thermal decomposition of the diazonium salts. According to the disclosure, the method has the advantage that either the batch method or continuous method can be used, and usually the reaction is carried out by continuously introducing a hydrofluoric acid solution of the compound that provides nitrous acid into a hydrofluoric acid salt or solution of the aromatic amino compound.
European Patent Application No. 205,019, to Riedel DeHaen AG, discloses a process for producing aromatic fluorides wherein aromatic diazonium fluoride is first prepared and then the aromatic diazonium fluoride is decomposed in a falling-film reactor. Typically, falling film reactors are vertical tubular reactors in which the reactor walls are heated to raise the temperature of the reactants. A rotating device in the center of the tube spreads the reactants in a thin film on the reactor walls. In the decomposition of aromatic diazonium fluoride, HF flashes off as the reactants move down the reactor walls. Unfortunately, it is difficult or impossible to control the temperature of the reactants. Consequently, in the decomposition of aromatic diazonium fluorides, the temperature of the reactants is allowed to seek its highest point, with the result that undesirable by products can form. Furthermore, the falling film reactor is mechanically complex and subject to by-product fouling. Another potential problem with the falling film reactor is that unreacted diazonium fluoride can build up in the reactor outlet. Large concentration build up of diazonium fluoride presents a safety hazard inasmuch as the diazonium fluoride can release a substantial amount of energy upon decomposition.
The heretofore known processes for preparing fluoroaromatic compounds via decomposition of corresponding aromatic diazonium fluorides have not been entirely satisfactory in that they typically are complex, inefficient, expensive, non-continuous and/or prone to result in an unacceptably high level of tar and/or other byproducts. Accordingly, there is a substantial need in the art for a continuous diazonium fluoride decomposition process which would overcome these disadvantages. There is also a need for a process for decomposing aromatic diazonium fluorides which provides for the facile control of temperature and concentration of the aromatic diazonium fluoride in order to optimize yield, minimize undesired by products and avoid unnecessary safety hazards.